Introduce per-host plugin support: Caddy module discovery, module
config endpoints, plugin schemas (basicauth, cloudflare) and the
host plugins API. On the frontend, add the Plugins page, plugin/host
plugin dialogs and schema-driven form fields.
Replace gorm.io/driver/sqlite (mattn/go-sqlite3, CGO) with
github.com/glebarez/sqlite (modernc, pure Go). Builds and tests no
longer need a C toolchain, fixing CI on runners without gcc. Drop the
race detector from CI (it requires CGO) and build a static binary in
Docker.
Route host changes through the selected Caddy provider, add auth routes
and the frontend-dir serving toggle, seed a default admin user on first
run, and validate space/comma separated domain lists via a custom
validator. Includes validator tests.
